1. Field of the Invention:
The present invention relates to a new polysilazane useful for the production of Si.sub.3 N.sub.4 /SiC ceramics, a molded article produced from said polysilazane, a process for producing said polysilazane, and a process for pyrolyzing said polysilazane into a ceramic material.
2. Description of the Prior Art:
Nowadays, silicon nitride, silicon carbide, and a mixture thereof are attracting considerable attention as ceramics materials. Research is being carried out to produce such ceramics materials, especially ceramic fiber, by pyrolyzing a polysilazane whose main chain is composed of nitrogen and silicon. According to known technology disclosed in Japanese Patent Laid-open No. 226890/1985, an organic silazane polymer is produced by reacting anhydrous ammonia with RSiHX.sub.2 in solution, thereby forming a cyclic or linear precursor, and reacting this precursor in the presence of a basic catalyst capable of deprotonating hydrogen from a nitrogen atom adjacent to a silicon atom, thereby forming the Si.sub.2 N.sub.2 bridge. This procedure is accomplished by, for example, blowing ammonia gas into methyldichlorosilane cooled in diethyl ether to bring about ammonolysis, filtering off the resulting ammonium chloride precipitates, and distilling away ether from the filtrate under reduced pressure, thereby obtaining a precursor having the skeletal repeating unit of the formula: ##STR2##
Having a number-average molecular weight of about 300, this precursor is considered to be a cyclic compound composed of about 5 skeletal repeating units. This precursor polymerizes, giving off hydrogen gas, when added, with stirring, dropwise to a tetrahydrofuran solution in which is suspended potassium hydride (KH). This is elucidated as follows by the inventors, Seyferth et al.
At first, the following reaction forms the amide functional group. ##STR3##
A pair of such functional groups join to each other to form a four-membered Si.sub.2 N.sub.2 ring, permitting the metal hydride to be regenerated.
Upon the addition of methyl iodide (CH.sub.3 I) to the reaction system after the completion of the following reaction: ##STR4##
the amide functional group reacts with methyl iodide, forming potassium iodide precipitates. ##STR5##
By centrifugally removing the potassium iodide precipitates and distilling away the solvent, there is obtained a polymer in the form of white powder. This polymer is found to have, for example, the following composition according to proton NMR and elemental analysis. EQU (CH.sub.3 SiHNH).sub.0.39 (CH.sub.3 SiHNCH.sub.3).sub.0.04 (CH.sub.3 SiN).sub.0.57
This composition may be represented by the following model. ##STR6##
The structure enclosed by the broken line denotes the residue unit of the precursor.
Unfortunately, the above-mentioned process has some disadvantages. That is, when the precursor is polymerized in the presence of a metal hydride catalyst, the crosslinking reaction proceeds to such an extent that the polymer gels into an insoluble jelly. This makes it difficult to control the molecular weight and also makes the reaction less reproducible. Thus, the resulting polymer is unstable and becomes insoluble as the result of crosslinking which proceeds with time.
Therefore, it has poor storage properties. In addition, the polymerization reaction has to be carried out carefully at a low temperature (below 30.degree. C.) because gelation takes place in a very short time if it is carried out at, say, 66.degree. C. (the reflux temperature of tetrahydrofuran). The polymerization reaction takes a long time before a desired molecular weight is reached.